Question

how much drug will remain at end of 6 hour when patient is given 200 mg drug 75mg excreted in 90 minutes and if drug follow first order kinetic

Answer 1

DRUG ABSORPTION A. Biologic Factors
1. Membrane structure and function
The cell membrane is a semipermeable lipoid sieve containing numerous aqueous channels, as well as a variety of specialized carrier molecules.
a. For most tissues, passive aqueous diffusion through channels
occurs only for molecules less than 150-200 MW. A notable exception is the endothelial capillary lining, whose relatively large pores allow molecules of 20-30,000 to pass. However, the capillaries of most of the brain lack these large pores.
b. Passive lipid diffusion is probably the most important absorptive mechanism. Lipid-soluble drugs dissolve in the membrane, and are driven through by a concentration gradient across the membrane.
c. Carrier-mediated facilitated transport occurs for some drugs, particularly those which are analogs of endogenous compounds for which there already exist specific membrane carrier systems. For example, methotrexate, an anticancer drug which is structurally similar to folic acid, is actively transported by the folate membrane transport system.
2. Local blood flow is a strong determinant of the rate of absorption because it continuously maintains the concentration gradient necessary for passive diffusion to occur. For orally administered drugs, remember that the blood supply draining the gut passes through the liver before reaching the systemic circulation. Since the liver is a major site of drug metabolism, this first-pass effect may reduce the amount of drug reaching the target tissue. In some cases, the first-pass effect results in metabolic activation of an inert pro-drug.
3. Gastric emptying times vary among patients and contribute significantly to intersubject variability in drug absorption.
4. Drug binding
Many drugs will bind strongly to proteins in the blood or to food substances in the gut. Binding to plasma proteins will increase the rate of passive absorption by maintaining the concentration gradient of free drug. For many drugs, the gastrointestinal absorption rate, but not the extent of absorption, is reduced by the presence of food in the gut. Some drugs are not affected by food, while the absorption of a third group of drugs is enhanced by food (bile secretion by liver in response to food in GI tract increases drug absorption). Some drugs are irritating and should be administered with meals to reduce adverse effects.